1. Field of the Invention
The present invention generally relates to an energy converter, and more particularly, to a new solar cell for converting solar light to electrical energy.
2. Description of the Prior Art
The problems associated with the development of efficient converters of sunlight (solar light) to electrical energy are well known. Although extensive research and development is conducted in this field, at very great expenditure, all known sunlight converters, generally referred to as solar cells, are relatively inefficient. Most of the known solar cells are of the photovoltaic p/n junction type. Such a cell includes a first layer of semiconductor material doped to one polarity, e.g., n, and a second counter-doped (p) semi-conductor layer, which together from the p/n junction. Most of the experience which has been gained in the development and fabrication of such cells, particularly in connection with space exploration, is with silicon (Si) as the doped semi-conductor material.
As is appreciated, sunlight includes energy in a spectrum of wavelengths rather than at one specific wavelength. Sunlight includes more photons of lower energy (longer wavelength) than photons of higher energy (shorter wavelength). The sunlight spectrum is such that the optimum band gap in electron volts (eV) is around 1.5eV. The band gap of silicon in terms of electron volts (eV) is about 1.1eV. Also, due to the limited band gap of Si, the output voltage of a silicon solar cell is low. It is generally on the order of half the silicon band gap, i.e., 0.6eV. These factors, among others, are the reasons why silicon solar cells are inefficient in converting solar light to electrical energy. The conversion efficiency of conventional p/n junction-type silicon cells is in the order of 11% of the incident energy at room temperature for space equivalent sunlight.
To increase the conversion efficiency, research is being conducted to develop a p/n junction-type cell in which gallium arsenide (GaAs) rather than Si is used. As is known, GaAs has a band gap of about 1.4eV which is closer to the optimum band gap for sunlight. Thus, the output voltage is expected to be higher than that with a silicon solar cell, due to the higher band bending taking place. However, despite the theoretical advantages of a p/n junction type GaAs cell, there are several problems which must first be solved before a more efficient cell can be produced.
With present day material technology, good quality GaAs of any of its ternary compounds can be produced. However, such materials are compound materials which tend to disassociate at the high temperatures, which are required for diffusing or annealing ion-implanted p/n junctions. Such disassociation of the material can adversely affect its diffusion length, which in turn would reduce the number of generated carriers contributing to the desired current. Thus, before GaAs (or its ternary compounds) can be used to fabricate a satifactory p/n junction type converter, advances in material technology must take place.
Also, even if such a cell were realizable at present, it would have several significant disadvantages. In a p/n junction type cell the top counter-doped layer is relatively thick, i.e., several thousand Angstrom units. Such a layer, made of doped wide band gap material would tend to absorb a significant amount of the sunlight, particularly the short wavelength of 0.8 microns and less. Also, at the top surface of the top counter-doped layer, the surface will provide recombination centers for the holes and electrons generated in the near surface region, thereby further reducing the carriers which would contribute to the generated current. It should thus be appreciated that even if the fabrication of p/n junction type cells with GaAs or any other semiconductor material of wider band gap (then silicon) were possible with present day technology, such a cell may produce limited current due to the discussed problems.